The present invention relates to a propulsion method of a pipe to be buried without soil discharge and to an excavator, and more particularly relates to a propulsion method without soil discharge capable of burying pipes progressively into a formed burying hole while drilling and forming a burying hole in the ground, without excavating the ground surface, when installing an underground pipe of sewer or the like, while treating the excavated soil inside the ground without discharging outside, and relates to an excavator for excavating a tunnel of a relatively small aperture in the ground, which is applied in said propulsion method.
As one of the propulsion methods of a pipe to be buried underground, the process of burying pipes progressively as the excavator having a drilling mechanism such as auger at the tip drills a burying hole in the ground and goes on excavating is known, and it is generally called the auger process.
In the auger process, usually, the soil excavated by the excavator is conveyed backward through the inside of the pipe row being propelled and buried, is brought up to the shaft or ground surface from the rear end of the pipe row to be buried, and is discarded. In this method, however, it is necessary to install a soil conveying mechanism inside the narrow burying hole from the excavator till the rear end of the pipe row to be buried, and the equipment is complicated, and the facility cost and running cost are high, and the propulsion speed of the excavator must be set by adjusting to the soil conveying capacity, and therefore the propulsion speed cannot be set so high, and it also takes labor and cost, among other problems, for discarding the excavated soil as refuse.
Accordingly in the case of a pipe to be buried of a relatively small diameter, the propulsion method without soil removal is employed, that is, the excavated soil is treated inside the burying hole and is not discharged outside. More specifically, a leading element shaped like cone etc. is pressed into the ground to form a burying hole. The soil put aside to the outer circumference by the leading element is compacted in the inner wall of the burying hole or the ground side, and therefore the burying hole may be formed without discharging the soil outside. Practical propulsion methods without soil removal are known to include the impact injection method by compressed air and injection method by hydraulic jack. In these methods, however, there is a very large resistance in injecting the leading element into the ground, and an extremely large propulsive force must be applied to the leading element. Accordingly, the facility of the jack for applying the propulsive force is increased in size, and a greater power is required in operation.
As a process without soil discharge to solve the above problems, the following process has been proposed. In this process, an excavating mechanism such as auger is attached to the tip of the leading element, and by this leading element of the excavator the ground on the front side of the excavator is drilled to form a burying hole corresponding to the outside diameter of the excavator, and the removed soil is once taken into the excavator, and the taken soil is forced out in the radial direction from the soil discharge port opened on the outer circumference of the excavator behind the excavator, and is compacted to the ground side from the inner wall of the burying hole. In this process, since the ground is first drilled by the excavator and the removed soil is compacted to the outer circumferential ground, as compared with the conventional process of injecting the leading element by force into the ground, the resistance in the axial direction is small, and it is possible to propel even with a relatively small propulsive force. In this process, moreover, a conical cone rotor rotating eccentrically is incorporated inside the excavator, and by forcing out the soil in the radial direction from the soil discharge port by the radial force due to the eccentric rotation of the cone rotor, it is also proposed to compact the soil to the ground side efficiently.
According to this improved propulsion method without soil charge, it is possible to bury pipes progressively at lower cost and more efficiently.
Even in the improved propulsion method without soil discharge, however, as the aperture of the pipe to be buried becomes larger, it is difficult to propel the excavator and the pipe row, and it cannot be applied to wide aperture pipes.
That is, in the process without soil discharge, as the aperture of the pipe to be buried becomes larger, the ground drilling diameter by the excavator is wider, and a massive soil of large drilling diameter must be compacted to the ground outside the excavator, and as the soil compaction volume increases, the resistance in propulsion increases, and a greater propulsive power is required, and if exceeding the tolerance of the compaction determined by the ground soil quality, it is no longer possible to compact, and propulsion is disabled.
This problem is explained in detail. As shown in FIG. 4, when burying a pipe with an outside diameter D, the soil in a range corresponding to the sectional area of .pi.D.sup.2 /4 must be completely compacted and discharged to the ground side. From the ground soil condition and the sideway compression capacity of the excavator, supposing the distance capable of compressing and expanding the inner wall of the burying hole to the outer circumference side in the ground section, that is, the possible compaction depth to be t, EQU .pi.(D+2t).sup.2 /4-.pi.D.sup.2 /4 (1)
is the gap that can be formed by sideway compression, that is, the sectional area of the space in which the soil can be accommodated, and the soil in the range corresponding to the sectional area of the outside diameter of the pipe to be buried must be discharged within this sectional area. In this case, assuming the volume decrease rate due to compaction of excavated soil to be .alpha., the soil in a range corresponding to the sectional area of EQU (1-.alpha.).pi.D.sup.2 /4 (2)
must be completely taken into the gap sectional area stated above. Accordingly, the following limit is applied to the inside diameter of the burying hole, that is, the outside diameter D of the pipe to be buried progressively. EQU (1-.alpha.).pi.D.sup.2 /4.ltoreq..pi.(D+2).sup.2 /4-.pi.D.sup.2 /4(3)
That is, EQU D.ltoreq.{2/[(2-.alpha.).sup.0.5 -1)]}t (4)
In other words, the outside diameter D of the pipe that can be propelled and buried without discharging soil is, supposing the sideway compression depth to be t and the volume decrease rate by compaction of excavated soil to be .alpha., possible up to {2/[(2-.alpha.).sup.0.5 -1]}t.
For example, in the general condition of conventional propulsion method without soil discharge, i.e., t=5 cm, .alpha.=0.1 (10%), a pipe of up to 26.4 cm in outside diameter D can be buried without soil discharge, but a wider pipe cannot be installed in the same process. If the compaction capacity is raised by using the eccentric rotation cone rotor to achieve t=5 cm, .alpha.=0.15, the maximum limit is D =44.6 cm as estimated from the above formula. In other words, in the case of pipes with medium or large diameter in which the excavated soil volume is large, its disposal cost occupies a large portion of the installation cost, and the merits of process without soil discharge are great, the propulsion method without soil discharge can be indeed rarely applied.